The invention relates to densitometers used in graphic arts to determine highlight, shadow and midtone densities necessary to reproduce photographs and similar graphic images with printing presses.
Procedures used in the past have been very labor intensive with painstaking attention being required to determine the criteria for reproduction of a graphic image. In the past, an individual technician knowledgeable in the art had to make a subjective determination of the brightest portion of a graphic image to be reproduced and of the darkest portion of a graphic image to be reproduced and, with a hand held densitometer, measure as precisely as he could position the densitometer the highlight density and the shadow density of the brightest and darkest portion of the graphic image. The determination of the tonal distribution, where to place the midtone dots, was even more subjective, variable from individual to individual.
Limited attempts at automating the Graphic Arts processes have been proposed. For example, gross measurements of the amount of light passed through a transparency has been proposed to determine the amount of ink required, on a per column basis, for offset printing. Using an appertured scanning bar, having discrete devices fixed in the apertures, data was obtained on a per column basis to determine how much of the image was dark or clear. This data was then used to supply the correct amount of ink which corresponded to the amount of image which was to be printed. The necessary alignment of the discrete devices through the apertures with targeted highlight and shadow densities, and other problems inherent in such a design, suggests that such a system would not be practical to make the density determinations which are achieved with the present invention.
Current technology lends itself to automation of the prior art processes. However, non-uniformity of light sensitive or light measuring devices and the problems of calibration and drift of such devices has been an obstacle to such automation. In addition, with hand held instruments positioned for a period of time the intensity of light could be regarded as uniform. At microprocessor based speeds, however, the fluctuating line voltage and cyclical nature of light becomes a problem which had to be overcome to automate densitometry. Finally, with a high resolution system, with measurement accuracy in the one to three millimeter range, methods had to be devised to accumulate and evaluate the volume of raw data that is obtained with an automated procedure.
Because of the magnitude of the problems involved in automating densitometry the manual method of locating the tonal distribution from highlight densities to shadow densities is the method that is in common use today.